An operating aircraft auxiliary power unit (APU) provides energy for functions other than propulsion. An APU generally consists of a gas turbine engine (hereinafter referred to as an “APU engine), a generator, and a load compressor (LC). Before the main propulsion engines are started, the APU engine is started, generally by a battery or hydraulic accumulator and fuel. The operating APU provides auxiliary electrical power and pneumatic power to the aircraft while the main propulsion engines are shut down, such as during aircraft ground operations and provides backup electrical and pneumatic power for in-flight operations. The operating APU supplies the electrical power via the generator which is driven by the operating APU engine. The activated load compressor in the operating APU supplies the pneumatic power for various aircraft systems and functions. These systems and functions may vary, and may include the aircraft environmental control system (ECS), the cabin pressure control system, and/or main propulsion engine start (MES) air.
APU efficiency is generally reported in terms of specific fuel consumption (SFC), the mass of fuel consumed per unit of energy output. Continued operation of the load compressor when pneumatic power is not needed (but the APU is still operating, albeit in “generator-only mode” to provide electrical power) results in a parasitic loss of fuel as well as undesirable air emissions. Therefore, it is beneficial to be able to deactivate the load compressor in the operating APU when pneumatic power is not needed, but to be able to (re)activate the load compressor in the operating APU when pneumatic power (auxiliary air) is needed. In a typical aircraft, for example, an APU Master Switch is turned to the “Start” position to initiate the APU start and is released to the “Run” position, which is the normal operating mode for an “operating APU”. When the APU Master Switch is turned to the “Off” position, the auxiliary air is automatically shut off and the APU shuts down. When the APU Master Switch is in the “Run” position and an APU Air switch is turned “ON”, the load compressor may be activated to provide compressed air (hereinafter “load compressor” or “LC” air) to the various aircraft systems and functions. When the APU Air switch is turned “Off”, the APU air shutoff valve closes to isolate the APU from the aircraft pneumatic system, and the load compressor is deactivated to standby mode. The APU Master switch turned to the “Off” position will shut down the APU regardless of the APU AIR switch position.
In a conventional operating APU, the activated load compressor is engaged with the operating APU engine and the deactivated load compressor is disengaged from the operating APU engine. The load compressor may be engaged with and disengaged from the operating APU engine via one or more friction clutches. Unfortunately, friction clutches that have been used to engage the load compressor with the operating APU engine and disengage the load compressor therefrom tend to be heavy and have less than ideal reliability in APUs due to high rotational speeds. High rotational speeds impose high inertial loading, leading to excessive temperatures and accelerated wear and failure of the friction clutch components, leading to clutch failures. In addition, oil-cooled friction clutches impose oil cooling system penalties. Dynamic disconnect clutches can also be used for disengaging the load compressor in the operating APU, but can only disengage at speed and cannot re-engage until the APU is shut down, thereby limiting operational flexibility.
Accordingly, it is desirable to provide auxiliary power units and methods and systems for activation and deactivation of a load compressor therein. In addition, it is also desirable to provide auxiliary power units in which the load compressor therein may be selectively engaged with and disengaged to the operating APU engine without a friction clutch, thereby resulting in a more lightweight and less complex APU, lower inertial loading and greater APU reliability. It is also desirable to provide methods and systems to selectively disengage the load compressor from the operating APU engine when pneumatic power is not needed, thereby improving aircraft fuel economy and reducing undesirable air emissions and to selectively engage with the operating APU engine when pneumatic power is needed, thereby maintaining operational flexibility. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.